NMR spectroscopy is a method of instrumental analysis, with which in particular the chemical composition of measurement samples can be determined. Thereby, radio-frequency pulses are emitted into the measurement sample, which is located in a strong, homogeneous static magnetic field B0, and the electromagnetic reaction of the sample is measured. With solid-state NMR spectroscopy, it is known to arrange the measurement sample at an angle that is tilted at the so-called “magic angle” of θm=arccos(√⅓)≈54.74° with respect to the homogeneous, static magnetic field, in order to reduce line broadening due to anisotropic interactions. This measurement technique is usually described as “magic-angle spinning” (MAS). The angle θm is a solution of the second order Legendre polynomial P2(cos(θm))=0, so that all interactions dependent on this Legendre polynomial disappear at this angle to the magnetic field. This is the case for three important interactions in solids: dipolar coupling, chemical shift anisotropy, and quadrupole interaction of the first order. As for polycrystalline measurement samples in which the crystal directions of individual crystallites are randomly oriented relative to the static field, the elimination of the interaction is achieved by a sufficiently fast rotation of the measurement sample at the magic angle. In this way, line broadening due to these interactions can be significantly reduced, ideally even to the natural line width.
NMR-MAS probeheads allow high resolution NMR spectroscopy to be carried out with solid, powder, or semi-solid (gel or paste) measurement samples. Thereby—as shown in FIG. 4—the measurement sample 5 is filled into a circular cylinder sample holder, the so-called rotor, which is rotated at very high speeds, with a rotation frequency in the range of a few kHz to over a hundred kHz, by means of compressed gases in a stator 10. The radial support is ensured by air bearings 20 in the stator, in the same manner as a holding force created by air flow holds the rotor in its axial position in the stator. The orientation of the rotation axis with respect to the static magnetic field is defined by the stator.
While for many NMR experiments in magnet systems with B0 fields in the range from 7 T to 25 T, setting of the magic angle with a precision from 0.1° to 0.01° is sufficient, for some applications, such as, for example, satellite transition (ST-MAS) NMR or proton spectroscopy, a precision of up to 0.001° is required. The angle setting should remain constant over a wide temperature range and be maintained in a reproducible manner when changing the measurement samples. This places extremely high demands on the mechanical components, if the setting is to take place in a controlled instead of in a feedback-regulated manner. Consequently, a measurement apparatus that measures the angle between the rotation axis and the static magnetic field in a reliable manner would allow the angle to be maintained in a feedback-regulated manner.
In general, these probeheads are used in superconducting NMR magnet systems, in which the homogeneous, static magnetic field B0 is oriented along a “bore hole,” which specifies the z-axis of the laboratory coordinate system. Alternatively, magnet systems in which the static magnetic field is oriented orthogonal to a bore hole of the magnet, can also be used. This is the case, for example, with permanent magnets or some superconducting horizontal magnets.
FIG. 4 shows a cross-section of the measurement sample 5, the rotation axis RA and the direction of the static magnetic field. Also shown are the air bearings 20, the drive 30, the tilt axis DA, around which the stator 1 can be tilted in order to adjust the angle of the measurement sample 5 to the static magnetic field, and the air supply lines 6 for pressurizing the air bearings 20 and the drive 30. Further elements of the NMR probehead, such as radio frequency (RF) coils, walls, networks etc., are not shown for the sake of simplicity. The rotation axis RA of the measurement sample 5 is also referred to as z′-axis and has a joint origin with the z-axis. The z- and z′-axes lie in one plane, which is spanned by the x- and z-axes and the x′- and z′-axes. The y- and y′-axes of the two coordinate systems are identical.
In the prior art, MAS probeheads generally comprise an adjustment mechanism, which allows precise setting of the angle θ between the rotation axis RA of the measurement sample along the z′-axis and the static magnetic field B0 along the z-axis. Such an adjustment mechanism that is integrated in the probehead is referred to as an “internal” or “integrated” mechanism. In general, the adjustment mechanism moves the measurement sample, the stator containing the bearing and the drive of the rotor, and the RF coils. This movement is induced by hoists, spindles and gear wheels, levers with linear movements or similar mechanisms, and primarily includes a rotation movement. However, generally rotation movements may also be combined with linear movements. Adjustment mechanisms with manual and motorized adjustment are known, particularly with electromotive adjustment. With many state-of-the-art probeheads, particularly those that are used in standard-bore magnet systems, i.e. magnet systems with a bore hole diameter of less than 60 mm, the angle adjustment can be carried out over a very large range, and also serves to facilitate removal of the measurement samples when changing samples. Probeheads that are tilted as a whole with respect to the magnet system to achieve the adjustment of the angle θ, are also known.
Furthermore, U.S. Pat. No. 8,547,099 B2 describes an NMR system with a probehead without adjusting mechanism. With this NMR system, the tilt of the rotation axis (z′-axis) with respect to the probehead and the magnet system is kept constant, and the direction of the static magnetic field is tilted by generating a field B1 with an additional electromagnetic coil. The additional electromagnetic coil is arranged around the measurement sample, so that the angle between the z′-axis and the direction of the linear combination of the B0 and B1 fields, correspond to the magic angle. Thus, with a probehead of such design, an electronic tilt of the angle θ is carried out.
Particularly when changing the sample temperature, removing or installing the probehead in the magnet system and changing measurement samples, the precision of the known adjustment mechanisms is often not sufficient for carrying out demanding NMR measurements. This particularly applies to proton spectroscopy and ST-MAS, where angle errors in the range of a few thousandths of a degree could lead to noticeable line broadenings in the measured spectra.
In the state of the art, the following method is used to adjust the angle θ between the rotation axis of the sample and the magnetic field direction: In general, a measurement sample (e.g. powdered potassium bromide) with the greatest possible dependency of the line width on the adjusted angle is measured via the NMR probehead. The line widths of the central line, and rotation side bands and/or the height of the lines and/or the ratio of amplitude/width between various lines are evaluated. Alternatively, an evaluation can be carried out directly on the time domain signal. Subsequently, the calibration measurement sample is removed and an actual measurement sample with the measurement substance is inserted in the probehead and measured using the angle setting from the calibration measurement.
In many cases this leads to errors, particularly if removing the probehead from the magnet system or tilting the stator to remove the rotor is required or a temperature change occurs between calibration and measurement.
MAS probeheads typically cover a very wide temperature range for the measurement samples. At the lower end of the temperature scale, there are probeheads that are designed for temperatures down to −50° C., −80° C., −130° C. or even for temperatures in the cryogenic area from 30K to 100K. In the upper temperature limit, values of temperatures up to +80° C., +150° C. or in the case of special samples even far beyond may be required. In most cases, the temperature of the measurement samples is ensured through use of a gas, whereby the air of the bearings and/or the drive air is also temperature-controlled to some extent.
Due to the compactness of the construction (the measurement sample diameters are typically in the range of 0.7 mm to 4 mm), the temperature of at least part of the tilt mechanism is close to the temperature of the measurement samples. Reproducing the adjustment of an angle with high precision and over a wide temperature range is technically extremely difficult to implement, and leads to high costs in the manufacturing of the mechanical parts.
Due to the complexity and difficulty of adjusting the angle θ of the rotation axis with respect to the z-axis in a precise and reproducible manner, there is a desire for a feedback-regulated adjustment, instead of the typical controlled adjustment using a calibration experiment but without feedback of the adjusted angle. In the prior art, three different concepts are known, which allow feedback between the adjusted angle and the measured angle:
U.S. Pat. No. 5,760,586 describes an MAS probehead, which includes automatic adjustment of the angle between the rotation axis of the measurement sample and the static field. One embodiment comprises a magnetic field sensor, in particular a Hall effect sensor for measuring said angle. Preferably, the Hall effect sensor is mounted in an inhomogeneous area of the static magnetic field and is moved by a mechanism within said area when the angle θ is adjusted in such a way that a calibration curve can correlate the adjusted angle θ with the measured magnetic field amplitude of the sensor.
U.S. Pat. No. 8,203,339 B2 describes an MAS probehead in which the adjustment of the angle between the rotation axis and the static magnetic field is regulated with a Hall effect sensor, whose orientation is as parallel as possible to the static field. The output signal (Hall voltage) is converted to an angle. Such a Hall sensor is shown in FIG. 4 with reference number 40.
The non-patent literature references titled “Optical alignment in magic-angle NMR” (G. Bodenhausen et al., J. Magn. Reson., 48 (1982), pp. 143-147) and “Optical lever for monitoring of the magic angle” (E. Mihaliuk, T. Gullion, J. Magn. Reson., 223, (2012), pp 46-50) describe a method of detecting the position of a laser beam reflected by the stator and using the detected position to determine the angle of the rotation axis with respect to the magnetic field.
The optical method is an indirect measurement of the angle. The signal does not directly depend on the magnetic field. Thus, it suffers from the disadvantage that the mechanical positioning of the probehead relative to the magnet field system affects the measured angle. Such a method is thus not suitable for delivering a measurement accuracy and reproducibility of 0.001°, particularly in the case where a probehead is removed.
The measurement of the angle using a Hall sensor is based on the Hall effect. This effect occurs for current-carrying conductors in a magnetic field and results in a voltage perpendicular to the current flow and field direction according toUH=(AHI/d)B⊥where UH is the Hall voltage, AH is the Hall coefficient, I is the control current, B⊥ is the magnetic flux density orthogonal to the sensor plane, and d is the thickness of the conductor.
FIG. 2A shows a top-view drawing (top) and a cross-section (bottom) of a planar Hall sensor, whose plane is orthogonal to the magnetic field. The crosses in the top-view drawing symbolize the direction of the static magnetic field B0. The Hall sensor has at least four contacts (1-4), whereby in FIG. 2A, a current flows between the contacts 1 and 3. The Hall voltage is established between the contacts 2 and 4 by the deviation of the electrons, whose paths are symbolized by dashed lines.
Unfortunately, in reality the Hall effect is less ideal than suggested by this formula.
The Hall coefficient AH depends on the temperature-dependent charge carrier density (electrons/holes) and the mobility of said charge carriers. This results in a marked temperature dependence of the Hall voltage. Additionally, with real sensors, a voltage caused by a combination of a temperature-related mechanical stress and a piezo-electric effect further distorts the temperature dependence of the Hall voltage.
In addition to the temperature dependence, effects depending on the magnetic field also occur. For example, the band structure of conductors generally changes in the magnetic field, which leads to non-linearities of the Hall effect, especially in strong magnetic fields.
Furthermore, Hall sensors generally experience an offset voltage in the zero field value, which occurs, for example, due to minor asymmetries of the contacts and/or positioning of the sensor with respect to the crystal axis of the substrate used. Furthermore, the Hall sensors may experience symptoms of aging, leading to long-term drifts. Thermoelectric voltages in the supply lines and contacts also lead to temperature-dependent errors due to an inadequate measurement assembly.
Moreover, additional magnetoresistive effects, such as the planar Hall effect (PHE), can be problematic for angle detection. With the planar Hall effect a voltageUPHE(APHEI/d)Bi2 sin(2φ)occurs, where φ is the angle between the projection of the magnetic field in the sensor plane and the control current. Additionally, APHE is only constant as a first approximation.
If a Hall sensor is exposed to a magnetic field that is not orthogonal to the sensor plane, then the output signal is the total of the Hall voltage and the planar Hall voltage.U=(AHI/d)B⊥+(APHEI/d)Bi2 sin(2φ)Furthermore, spin-dependent scattering processes can occur (abnormal Hall effect) during operation at cryogenic temperatures, as well as Shubnikov-de Haas and Quantum-Hall effects.
Thus, precision measurements require careful calibration over the entire application range (magnetic field strength, angle and temperature), which has to be repeated at regular intervals to correct aging effects. In addition, it is desirable to use only sensors in which the offset voltages are already reduced as far as possible due to the design and “perturbations” such as the PHE are kept to a minimum. For example, the PHE of a Hall sensor based on a heterojunction is up to 50 times smaller than the PHE of a comparable Hall sensor from a single, continuously doped semiconductor.
The methods known from the prior art for determining and adjusting the magic angle of NMR probeheads using a magnetic field measurement are based on two different principles: In U.S. Pat. No. 8,203,339 B2, a Hall sensor is mounted in the static magnetic field in such a way that the sensor plane and thus the orientation of the sensitivity vector is as parallel as possible, i.e. at an angle below 5°, to said sensor. The obtained Hall voltage is then converted into an angle and used for adjusting the angle.
This approach has several problems:                1. If the sensor is in a parallel orientation to the field, it is operated in a range in which the PHE can lead to comparable or even higher voltages than the normal Hall effect. The angle measurement is thus no longer primarily dependent on the orthogonal component of the magnet field, but on the angle of the current to the parallel component of the field.        2. The adjustment of the angle depends directly on the accuracy of the measurement of the Hall voltage and measurement current.        3. Any drift and changed Hall coefficients (temperature dependence) result in angle errors, unless the angle of the sensor is adjusted to exactly 0°.        
U.S. Pat. No. 5,760,586 also uses a Hall sensor as a magnetic field sensor. However, the hall sensor is positioned in an inhomogeneous area of the magnetic field. A rotation movement of the stator defining the rotation axis of the measurement sample is turned into a translation of the sensor in the inhomogeneous field via a mechanism. The obtained Hall voltage is then converted into an angle via a calibration table and used for adjusting said angle.
This approach poses the following problems:                1. A position inaccuracy along the z-axis results in angle errors. This inaccuracy can occur, for example, if the probehead position is referenced with respect to a shim system, but said shim system changes its length relative to the fixing point at the magnet due to temperature variations.        2. The inhomogeneous field depends on the adjusted currents of the shim system. Fluctuations of the shim fields due to the fluctuations of the shim current sources are also transferred directly to fluctuations of the measured angle.        3. The adjustment of the angle depends directly on the accuracy of the measurement of the Hall voltage and measurement current.        4. Any drift and changed Hall coefficients (temperature dependence) result in angle errors.        